This invention relates to x-ray apparatus, more particularly to an improvement in the cathode electrode for x-ray tubes.
The cathode electrode of an x-ray tube generally comprises one or more helical wire filaments each forming a cylinder, and supported in spaced relationship within an electrically conductive cathode cup. A small voltage impressed across the filament causes filament current to flow and provide a source of electrons; the filament and electrode cup are generally kept at or near the same electrical potential in the x-ray tube.
An anode electrode, which may be stationary or rotating, is positioned within the x-ray tube and a relatively large electrical potential is impressed between the anode and cathode causing electrons generated by the filament to strike the anode in a predetermined area, the image of which is called the focal spot.
The location of the filament in the cup or more particularly, in a focusing slot in the cup, and the shape of the slot determines in part the size and emission, or x-ray generating capability, of the focal spot.
In the past it has been considered desirable for x-ray tubes intended for mammography applications to have a focal spot substantially less than 0.3, in particular, a 0.1 nominal focal spot. The size of x-ray tube focal spots is conventionally identified by reference to a dimensionless number which correlates to the width of the focal spot in millimeters as will be explained more fully below. Prior art tubes which have attempted to achieve such small focal spots have exhibited undesirably low emission levels and unacceptable growth or "blooming" in focal spot size as a function of emission current. For example, a prior art tube has been observed to have emission levels of only 8 to 13 mA of anode-cathode current while exhibiting nearly a two-to-one variation or blooming of a focal spot from 0.185 to 0.36 mm.
To achieve the small focal spot sizes needed for mammography, prior art cathode electrode focusing cups have placed the filament relatively deeply within the back or rear slot of the focusing cup. In addition, relatively close dimensions, for example 0.005 inches spacing between the filament and each side wall of the back slot has been observed in such prior art designs. With such small tolerances, a relatively small movement of the filament could result in shorting of the filament to the side wall, resulting in a tube failure. Furthermore, such a prior art structure severely limits the electron emission of the filament and thereby reduces the anode target loading and ultimately results in poor x-ray emission produced by the tube. The limited emission of this prior art design is believed to be the result of a space-charge-limited mode of filament operation.